Polypeptides are an important class of biopolymers, not only serving as living biological system's structural and functional carrier, but also having broad applications in the fields of materials, catalysis and pharmaceuticals. Traditionally, polypeptide synthesis has been almost exclusively based on the use of amino acids as starting materials, which requires tedious procedures for presynthesis of amino acids and subsequent activation of the highly stable carboxyl groups, using stoichiometric amounts of special reagents, in order to form peptide bonds. Inside biological systems, this kind of synthesis is carried out by the ribosome and catalysis of relevant enzymes. Using this method, polypeptides with specific amino acid sequences can be synthesized by genetic engineering, but the method is basically limited to using natural amino acids as starting materials. Using a chemical method, synthesis of polypeptides would not be limited by the starting materials. Such methods include, for example, Liquid-Phase Coupling methods and Solid-Phase Synthesis techniques, and both are frequently used for preparation of oligopeptides, but they involve tedious steps and relatively high cost and are not suitable for the synthesis of polypeptides with high molecular weights.
For synthesizing high molecular weight polypeptides, however, the most efficient method should be the Ring-Opening Polymerisation of amino acid-N-carboxyanhydrides (NCA). This method also starts from amino acids and hence involves relatively high cost, which hindered large-scale production and application of the polypeptide materials. In addition, the Ring-Opening Polymerization method is usually used to synthesize polypeptides composed of N-unsubstituted amino acids and is rarely employed to synthesize polypeptides of the N-substituted amino acids because of steric hindrance (Ballard et al., J. Chem. Soc., 355 (1958)). To date, the only polypeptides of the later kind that have been obtained are those made from proline and sarcosine, as well as the short polypeptides from N-methylalanine (Cosani et al., Macromol., 11, 1041 (1978)).
Polypeptides composed of the natural occurring N-unsubstituted amino acids are normally water-soluble, unsuitable for direct use as such biomedical materials as those used for artificial organs. By contrast, N-substituted polypeptides, in which the water-soluble —NH functional groups have been removed, can be insoluble in water. However, such polypeptides have long been difficult to prepare due to lack of effective synthesis methods. One of the most attractive possible ways to solve the problem is to find a method to directly construct polypeptides through copolymerization of imines and carbon monoxide. Unfortunately, such a reaction has not been achieved due to the lack of suitable catalysts.
In 1998, Sen and Arndtsen reported independently the insertion of imines into an acyl-Pa bond, which represents one of the key steps for imine and carbon monoxide copolymerization. Although they were not able to obtain the expectated polypeptides, they successfully obtained amides (Kacker et al., Angew. Chem. Int. Ed., 37, 1251 (1998); Dghaym et al. Organometallics, 17, 4 (1998)). This is the first example that an imine was successfully inserted into a transition metal—carbon bond. Arndtsen et al. have also tried the possibility of using nickel and manganese carbonyl complexes to catalyze the imine insertion, but up to now the best result obtained is the formation of an amino acid unit from the single insertion of an imine and a carbon monoxide molecule into a metal—carbon bond. In the efforts towards polypeptice synthesis, even a simple dipeptide has never been obtained successfully by this method. (Davis et al., Organometallics, 19, 4657 (2000); Lafrance et al., Organometallics, 20, 1128 (2001)).
It is noted that cobalt is an important catalyst for carbonylation reactions, and has been used to catalyze copolymerization of aziridines and carbon monoxide. (Jia et al., J. Am. Chem. Soc., 24, 7282 (2002)). However, when Sen et al. tried to use such catalysts to catalyze the copolymerization of imines and carbon monoxide, they failed to obtain the expected polypeptide products, but instead obtained N-alkyl phthalimidine (Funk, et al. Helv. Chim. Acta, 89, 1687 (2006)).
Thus, the primary focus of the present invention is the finding of an effective catalyst that can be utilized to realize the copolymerization of imines and carbon monoxide to synthesize polypeptides with unique, unexpected properties, which were not obtainable previously.